Electrolyte additive

ABSTRACT

An electrolyte for use in a lithium ion cell that has a tin anode, the electrolyte comprising vinyl ethylene carbonate.

This invention relates to an electrolyte for a lithium ion cell in whichthe anode comprises tin, and to lithium ion cells containing such anelectrolyte.

For many years it has been known to make cells with lithium metalanodes, and cathodes of a material into which lithium ions can beintercalated or inserted. A wide variety of intercalation or insertionmaterials are known as cathode materials for rechargeable lithium cells,such as TiS₂ or V₆O₁₃. To avoid the problems arising from dendritegrowth at lithium metal anodes during cycling it has been proposed touse an intercalation material such as carbon as the anode material. Inthis case the cathode material will be generally an intercalationmaterial that initially contains lithium ions, such as Li_(x)CoO₂ wherex is less than 1. Rechargeable cells of this type, in which both theanode and cathode contain intercalated lithium ions, are now availablecommercially, and may be referred to as lithium ion cells, or as swingor rocking-chair cells. Several different carbonaceous materials such ascoke, graphite or carbon fibre have been suggested for use in anodes.Graphite is commonly used commercially, but the capacity of thismaterial in commercial cells is close to the theoretical limit for LiC₆(372 mA h/g). Alternative anode materials have therefore been suggestedin order to increase electrode capacity, and in this respect tinelectrodes have the benefit of a markedly higher theoretical capacity:994 mA h/g for Li_(4.4)Sn. However, during insertion of lithium ions,very large volume changes occur, which lead to breakup of the electrodematerial and so poor cycle performance.

Annealing a tin electrode before use so as to form, for example, atin-copper alloy has been found to improve the cycle performance of thecell. However, such electrodes still have lower cycle efficiencies thanare required for long cycle life Li-ion cells.

The present invention aims to address this problem of poor cycleperformance for cells with tin anodes.

Accordingly, the present invention provides an electrolyte for use in alithium ion cell comprising a tin anode, the electrolyte comprisingvinyl ethylene carbonate.

The present invention also provides a lithium ion cell with a tin anodewherein the electrolyte comprises vinyl ethylene carbonate.

Typically the electrolyte comprises from 0.5 to 20 volume % of vinylethylene carbonate, preferably from 1 to 10 volume % and most preferablyabout 5 volume %.

The electrolyte may also comprise ethylene carbonate, propylenecarbonate, diethyl carbonate, dimethyl carbonate or mixtures thereof. Ina preferred embodiment the electrolyte comprises ethylene carbonateand/or propylene carbonate.

The electrolyte must also contain lithium ions and so comprises alithium salt such as LiPF₆, LiBF₄, lithium imide (LiN(CF₃SO₂)₂ orlithium bis-(trifluoromethanesulphonyl) imide) or lithium methide(LiC(SO₂CF₃) 3, lithium tris-(trifluoromethanesulphonyl) methide.

In a preferred embodiment, the electrolyte comprises vinyl ethylenecarbonate, ethylene carbonate, propylene carbonate and a lithium salt.

The invention also provides a process for making a lithium ion cell witha tin anode which process comprises: making an anode comprising a layerof tin; assembling a cell comprising the said anode, a cathodecomprising lithium ions, and an electrolyte comprising lithium ions andvinyl ethylene carbonate.

The cathode is made of a material containing intercalated lithium ions.For example, a lithium cobalt oxide, a lithium nickel oxide, a lithiumnickel cobalt oxide such as LiNi_(1-x)Co_(x)O₂ or a lithium manganeseoxide such as LiMn₂O₄.

The present invention also provides the use of an electrolyte comprisingvinyl ethylene carbonate and a lithium salt in a lithium ion cellcomprising a tin anode.

The invention will now be further described by way of example only, andwith reference to the accompanying drawing which shows a graph ofefficiency against cycle number for the test cells described in Example1 and Comparative Example 1.

EXAMPLE 1

Test cells containing a tin anode were made containing an electrolyteconsisting of ethylene carbonate, propylene carbonate, 1 molar LiPF₆ andvinyl ethylene carbonate. The ratio of ethylene carbonate to propylenecarbonate was 2:1 by volume. The ethylene and propylene carbonates weremixed in a 2:1 ratio and then vinyl ethylene carbonate was added to form5 volume % of the final mixture. LiPF₆ was added to the mixture ofliquids to form a 1 molar solution. The test cells contained threeelectrodes: a LiCoO₂ counter electrode and a lithium reference electrodeas well as the tin anode.

The tin electrode was cycled with respect to the lithium referenceelectrode between voltage limits of 0.01 V and 2.00 V at a constantcurrent.

The cycle efficiencies of the three cells (D, E and F) are shown in FIG.1.

COMPARATIVE EXAMPLE 1

Test cells were made as described in Example 1 containing an electrolyteconsisting of a 2:1 ratio of ethylene carbonate to propylene carbonateto which was added LiPF₆ to form a 1 molar solution.

These test cells were cycled between voltage limits as described inExample 1 and the cycle efficiencies of the three cells (A, B and C) areshown in FIG. 1.

From FIG. 1 it can be seen that an increased cycle efficiency is shownby the cells where the electrolyte comprises vinyl ethylene carbonate.Thus a tin anode lithium ion cell where the electrolyte comprises vinylethylene carbonate will last for a greater number of cycles than a cellwithout vinyl ethylene carbonate.

1. An electrolyte for use in a lithium ion cell comprising a tin anode,the electrolyte comprising vinyl ethylene carbonate.
 2. An electrolyteaccording to claim 1 wherein the electrolyte comprises from 0.5 to 20volume % of vinyl ethylene carbonate.
 3. An electrolyte according toclaim 2 wherein the electrolyte comprises 5 volume % of vinyl ethylenecarbonate.
 4. An electrolyte according to claim 1 wherein theelectrolyte further comprises ethylene carbonate and propylenecarbonate.
 5. A lithium ion cell with a tin anode wherein theelectrolyte comprises vinyl ethylene carbonate.
 6. A cell according toclaim 5 wherein the electrolyte comprises from 0.5 to 20 volume % ofvinyl ethylene carbonate.